The vast majority of electrical power transmission and distribution systems use alternating current, e.g., operating at 50 or 60 Hz. These AC systems form large interconnected networks spanning from the points of power generation to the points of power consumption and represent varying transformer-converted voltages at different parts within the system. Characteristically, the transmission portions of such systems operate at high AC voltages, to provide more efficient power transmission.
While raising the transmission voltage in such systems offers certain advantages, AC transmission has inherent problems, including the power losses associated with cyclically charging and discharging the transmission lines at the operating frequency of the system. Other issues inherent to AC transmission include the “skin effect,” which describes the tendency for AC current to flow near the outer surface or skin of a solid conductor. For these and other reasons, High Voltage Direct Current, HVDC, power transmission offers compelling advantages in a number of scenarios.
For example, HVDC transmission generally will offer economic advantages where overhead transmission line lengths are >600 km, where undersea cable lengths are >50 km, where underground transmission lines are required, where AC systems of different voltages and/or frequencies require interconnection, and where precise control of transmitted power is required. As a further key advantage, HVDC interconnections provide “firewalling” between the AC systems coupled through such interconnections, thereby preventing the failure of one AC grid from catastrophically cascading into other AC grids.
ABB is a pioneer in HVDC transmission, deploying the “Gotland” link in Sweden in 1954 using a version of “HVDC Classic” technology. Converters based on HVDC Classic employ thyristors as line-commutated current source converters, for converting from AC to DC, and vice versa. DC voltage levels and the direction of power flow are controlled by controlling the firing angle of the thyristors, which are typically arranged as “valves” that comprise stacked arrays of thyristors.
FIG. 1 illustrates a generic, known arrangement for interconnecting two AC power systems 10 and 12 via an HVDC transmission link 14 that is connected on one end to the AC power system 10 by a “converter” station 16 and on the other end to the AC power system 12 by a converter station 18. On the AC side, the converter station 16 connects to the first AC power system 10 via transformer(s) 20. Likewise, the converter station 18 connects on its AC side to the AC power system 12 via transformer(s) 22. These converter stations are, for example, based on HVDC Classic technology and FIG. 2 illustrates the use of thyristors 30 in such embodiments of the converter stations 16 and 18, which also may be referred to simply as “converters.”
Among their several advantages, HVDC Classic systems are robust and, with ongoing refinement and development, they offer the ability to operate reliably at high DC voltages, e.g., >500 kV. HVDC Classic systems are not without challenges, however, including those related to reactive power consumption and harmonics generation on the AC side of such converters.
Other technologies are known. For example, ABB also offers so-called HVDC LIGHT technology, which complements the use of underground transmission cables and provides power transmission in the range of 1,200 MW at DC voltages of +/−320 kV. HVDC LIGHT systems use voltage-source controlled converters that use arrays of Insulated Gate Bipolar Transistors or IGBTs. FIG. 3 illustrates a converter 32 based on IGBT switches, as an example alternative implementation for the converters 16 and 18. However, the challenges involved in scaling such technologies for operation at higher voltages means that higher DC voltage installations generally favor the use of HVDC Classic converters.